Active edge roll control in a glass drawings process

ABSTRACT

An apparatus for drawing a glass ribbon including an edge roll assembly that contacts the glass ribbon with a force that is dynamically altered by an actuator electrically coupled to a sensor that measures the force applied against the ribbon. Dynamic, or real-time, variation of the edge roll force minimizes stress variability in the glass ribbon and improves ribbon shape control.

CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 61/293,364, filed on Jan. 8, 2010. The content of this document andthe entire disclosure of publications, patents, and patent documentsmentioned herein are incorporated by reference.

TECHNICAL FIELD

This invention is directed to an apparatus and method of controlling thestress in glass ribbon produced by a glass drawing operation by activelyaltering the force exerted against the glass ribbon by edge rolls thatcontact the ribbon.

BACKGROUND

One method of forming a thin sheet of glass is by a drawing processwhere a ribbon of glass is drawn from a reservoir of molten glass. Thismay be accomplished, for example, via an up-draw process, where theribbon is drawn upward from the reservoir (e.g. Foucault or Colburn), orby a down-draw process (e.g. slot or fusion), where the ribbon is drawndownward, typically from a forming body. Once the ribbon is formed,individual sheets of glass are cut from the ribbon.

In a conventional downdraw process, such as a fusion downdraw process,the molten glass is formed into a glass ribbon contained within a drawchamber defined by a shroud that surrounds the ribbon. Among otherthings the shroud serves to maintain a consistent thermal environment inthe region defined by the shroud and surrounding the ribbon. Rollerpairs penetrate the shroud and pinch the ribbon edges. The rollers (orrolls) may be used to apply a pulling force to the ribbon, to apply atransverse tension to the ribbon, or merely to guide the ribbon.Accordingly, a rotational force may be applied to the rolls by a motor,or the rolls may be freewheeling and the rotational force applied to therollers by the descending ribbon. In either case, the rolls rotate.Production roll mechanisms typically allow for the rolls to movehorizontally and/or vertically from the glass contact area. Thisaccommodates geometric tolerances of the rolls, run out and tolerancechanges in operation, along with normal variability in glass thickness.Further, production roll mechanisms typically allow the rolls to movefar away from the glass for maintenance access, process restart, andother practical considerations. However, the rolls must also accommodatethickness changes at the edges of the ribbon, or dimensional variationsin the edge rollers themselves. Frictional forces that resist the freemotion of the edge rolls traverse to the ribbon during productionoperation may induce force cycling that manifests as undesirableperturbations or stress changes in the ribbon that can become frozeninto the glass as the glass transitions from a viscous material to anelastic material.

SUMMARY

Improvements in roll drive systems for downdraw glass manufacturingsystems enable significantly improved attributes, longer processlife—and will be especially beneficial draw where the glass ribbonproduced by the process is less stiff—meaning wider platforms, andthinner and taller ribbons. The ability to prescribe the tension forcesin the visco-elastic area of the ribbon, reducing the cycling of theseforces can facilitate significant benefit to ribbon attributes,especially warp and stress in the ribbon.

One issue to be addressed is that current roll systems define a pinchforce—that pressures the rolls together—and causes them to generate across-draw (across the ribbon) tension (or compression) effect. Thispinch force can vary with equipment setup due to a variety of frictionalmechanical issues—and can vary with roll wear or due to the thermalenvironment changes or with jamming of the equipment by foreignparticles (e.g. glass particles) in the environment.

A pinch force shift causes the cross-draw tension produced by the rollsto change. A consistent roll tension is important to shape and stressesin the ribbon, and the separate glass sheets or panes later separatedfrom the ribbon.

Another issue is that the current roll systems show a cycling of forcesthat occurs over the rotation of the roll pairs. This rotationalvariability manifests as a cycling of tension force and pulling forces(when the rolls are driven). In addition, this roll pair cyclinginteracts between the various roll pairs. Effects from this forcecycling are imprinted into the glass—resulting in warp and stressvariation—and also contribute to process stability. For example, changesin force can cause the ribbon shape to ‘pop’. That is, to instantlychange.

The dominant source of the roll force cycling occurs from mechanical runout between the roll pairs at opposite sides of the ribbon. This can becaused by the out-of-roundness of each roll, by the run out of thebearings, if the axles are not straight and parallel to each other ordifferences in the roll material compressibility or other propertiesover its circumference. Since the roll pairs have run out, the rollmounting mechanism must allow for the axles to move apart as the rollsrotate. Though attempts are made to minimize pinch force variation, thispinch force cycle, in turn, causes the cross-draw tension force and whendriven, the pulling force, to vary in a cyclical pattern.

To avoid this cycling affect, one can either eliminate the roll run out(which has practical limits), eliminate friction in allowing the rollsto move, or to automatically counteract the frictional effect—byapplying an offsetting force to counteract the cycling frictional forces

Further, roll force cycling results from the variable pulling load asseparate sheets are removed from the ribbon. The change in ribbonweight, for example, has a discernable impact on the roll forces. Also,force changes are observed as the sheet is cutoff from the separationprocess equipment. This is due to the forces imparted to separate thesheet, in addition to the force change simply from removing the sheetweight. These sheet cycle forces are different than the roll rotationaleffects described above. The sheet cycle forces interact with the rollsystems and impact the product.

Thus, a means to precisely manipulate tension force levels from a sheetdraw roll pair are described herein. This tension is important toproduct sheet shape and stresses. A manual or automated control schemeis can be used to tune roll tensions to optimal levels for productattributes. In the case of a roll that extends across the width of theribbon, or in some other specific applications, tension may not be ableto be directly measured. In this case, a method to measure and controlthe pinch force is disclosed as a suitable surrogate to roll tensionmanipulation.

The apparatus described herein can be used to eliminate sheet draw rollforce cycling from geometric run out or material property variationusing an automated system which automatically varies the applied pinchforce to cancel out (or offset) the variability and maintain a constantcross-draw tension and/or constant pulling force. As an alternative, apinch force sensor can be used. By actuation of the roll pair pinchforce, or another method such as actuation of down tilt angle or contactangle, along with sensing of the roll tension, the consistency oftension can be dramatically improved—in essence giving a constanttension to the glass process.

Other means to adjust roll tension such as varying the roll down tiltangle or by varying the roll/ribbon contact angle (both in the case ofcantilevered rolls) for this invention are practical

The methods described herein can result in improved flatness (lack ofwarp), and lower stress (from lack of product shape). This is especiallyimportant for ribbon draw processes where the glass ribbon lacksstiffness, either because a very thin ribbon is being produced, or ifthe ribbon is tall and/or wide, or if the process thermal conditions donot allow manipulation of the thermal profile (down and across theribbon) to maintain high thermally induced tension. The apparatus andmethods described herein can, for example, limit lateral (across a widthof the ribbon) tension variation and/or the perpendicular (normal) pinchforce exerted by the edge rolls against the ribbon due to such forcecycling to less than 4.5 kg peak-to-peak variation, preferably less than3.0 kg.

An automated control scheme can be constructed using known PiDapproaches or anticipator control schemes that will eliminatevariability from roll force cycling and variability of roll load comingfrom the separation and removal of the glass sheets.

Full-length edge rolls or cantilevered rolls can be used. However, insome implementations a sensor for measuring tension may not bepractical. In this case, a pinch force (or normal force) sensor can beused for a control target.

A practical actuator can be made from a variety of options. A linearservomotor has been found to perform this function well, although avariety of other options are possible. A linear servo is used to providea part or total of the roll pinch (or pressuring forces). In someembodiments the servo is added to counterweight linkages, which providethe pressuring function for the rolls.

A bleeding compressed air cylinder can also be used as an actuator, aswell as linear motors. A design consideration is the overall forcecapability of the actuator, along with a fast response time if theactuator is used to counteract short-term variability.

The optimal roll force levels can be defined by experimentation or byoffline modeling. It is believed that the most comprehensive means is toconduct experiments for roll forces to optimize the forces that givebest product attributes for a particular production setup. Methods likeDOE or Evolutionary Operations could be applied to define the optimalconditions for roll tensions. For cantilevered rolls it is believed thetotal tension of the roll pair, that is, the sum of right and left axlesensors, is the most important to product performance. However, theoptimal tension for each roll pair should be determined independently.

More sophisticated optimization approaches can also be used, such aswhere the roll forces are optimized in concert with the thermal setup ofthe draw. Concurrent optimization strategies can be helpful in definingoptimal roll tension force levels.

Once target tension levels are defined for each roll pair, the actuatorand sensor system can be used to hold to optimal levels, therebycounteracting normal process drift of the forces.

Another approach is to compensate for short-term roll force variabilityresulting from the roll rotation and variable pulling load from thesheet removal process. This can be combined with the methodology aboveto hold the tension at an optimal target, or can be used to simplyeliminate cycling forces.

Accordingly, in one embodiment, an apparatus for drawing a ribbon of aninorganic glass is disclosed comprising an edge roll assembly comprisinga rotatable shaft and an edge roll coupled to the shaft that contacts anedge of the glass ribbon. The apparatus further comprises a sensor thatdetects a tension force or a pinch force and develops an electricalsignal proportional to the tension force or pinch force, a controllerthat receives the sensor signal and develops a corrective signal and anactuator coupled to the edge roll shaft that receives the correctivesignal from the controller and varies the tension force or the pinchforce in response to the corrective signal. The apparatus may beconfigured to move the edge roll shaft in a horizontal plane or avertical plane.

In some embodiments, at least a portion of the applied pinch force is apassive force. That is, a force that is not actively varied. Examples ofpassive forces are gravity (i.e. acting through dead weights) andsprings. In comparison, an active force is a force that can be activelyvaried, such as a change in magnitude as a function of time. An exampleof an active force is a force that is applied by an actuator such as amotor, pneumatic or hydraulic piston, solenoid, and the like. In someembodiments, the total pinch force is supplied as an active force suchas through an actuator. In other embodiments the total pinch force isthe sum of a passive and an active pinch force.

In another embodiment, a method of making a glass ribbon is describedcomprising producing a glass ribbon in a down draw glass making process,the glass ribbon comprising a visco-elastic region, contacting thevisco-elastic region of the glass ribbon with opposing rollers, theopposing rollers applying a pinch force and a tension force on the glassribbon and sensing a magnitude of the pinch force or a magnitude of thetension force and producing a signal representative of the sensedmagnitude of the pinch force or the tension force. The produced signalis compared with a predetermined set point, and a corrective signal isgenerated. The corrective signal may then be used to drive an actuatorthat varies the pinch force or the tension force applied to the glassribbon by the opposing rollers so that the applied pinch force ortension force is substantially equal to the set point. The pinch forcemay comprise a passive force in addition to an active force.

In some embodiments the actuator translates a shaft of at least one ofthe opposing rollers in a direction perpendicular to a longitudinal axisof the shaft. In another embodiment the actuator rotates a shaft of atleast one of the opposing rollers through an angle lying in a horizontalplane. The actuator may rotate a shaft of at least one of the opposingrollers through an angle lying in a vertical plane.

The invention will be understood more easily and other objects,characteristics, details and advantages thereof will become more clearlyapparent in the course of the following explanatory description, whichis given, without in any way implying a limitation, with reference tothe attached Figures. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the present invention, and be protected by theaccompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front elevation view of an exemplary fusion downdrawprocess according to an embodiment of the present invention.

FIG. 1B is an edge view of a portion of a glass ribbon formed via adowndraw process, wherein an edge of the ribbon is engaged between apair of opposing edge rolls.

FIG. 2 is a cross sectional view showing a pair of edge roll assembliesengaged with a glass ribbon wherein a pinch force is applied by both apassive force and an active force.

FIG. 3 is a side view relative to a glass ribbon wherein pinch force isapplied through a rotary motion by both a passive force (dead weight viagravity G) and an active force (actuator).

FIG. 4 is a perspective view of an edge roll assembly according to anembodiment of the present invention.

FIG. 5 is a side view of the edge roll assembly of FIG. 4 showing aportion of the interior thereof.

FIG. 6 is a bottom view of a portion of the edge roll assembly of FIG. 4showing connecting webs for measuring force.

FIG. 7 is an end view of the embodiment of FIG. 4 showing dovetailcoupling between upper and lower (frame) support assembly portions.

FIG. 8 is a side view of the embodiment of FIG. 4 showing the edge rollhousing and shaft in a tilted relationship relative to a horizontalplane.

FIG. 9 is a perspective view of the embodiment of FIG. 4 illustratingthe coupling between the upper support and the frame.

FIG. 10 is a top view of the frame of FIG. 9 showing flexure of thewebs.

FIG. 11 is a side view of the frame of FIG. 9 showing a sensor andsensor target, an actuator, controller and signal communication linesfor moving the shaft of the edge roll relative to the ribbon of glass.

FIG. 12 is a top view of another frame configuration according toanother embodiment of the present invention.

FIG. 13 is a side view of the frame of FIG. 12 showing rotation of theframe.

FIG. 14 is an embodiment of the present invention illustratingtranslational movement of an edge roll shaft in a horizontal plane toadjust pinch force and or tension via an actuator coupled to the shaft.

FIG. 15 is an embodiment of the present invention illustrating angularmovement of an edge roll shaft in a horizontal plane to adjust pinchforce and/or tension via an actuator.

FIG. 16 is an embodiment of the present invention illustrating angularmovement of edge roll shafts in a vertical plane.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

Drawing a thin ribbon of material to form a glass sheet having athickness less than about a millimeter to the exacting standards offlatness required for modern display applications, such as televisionsand computer monitors, requires careful control of all aspects of themanufacturing process. However, particular attention must be paid to theperiod of time during which the glass ribbon is transitioning from avisco-elastic state to a solid elastic state. Even small forcevariations on the ribbon, such as might be produced by air currents inthe drawing area, or vibrations from running equipment, can manifest asperturbations in what should be a pristine, flat surface, and may induceresidual stresses in the ribbon that are retained by the eventual sheetseparated from the ribbon.

In an exemplary fusion-type downdraw process, molten glass is suppliedto a forming body comprising a channel open at its top formed in anupper surface of the body. The molten glass overflows the walls of thechannel and flows down converging outside surfaces of the forming bodyuntil the separate flows meet at the line along which the convergingsurfaces meet (i.e. the “root”). There, the separate flows join, orfuse, to become a single ribbon of glass that flows downward from theforming body. Various rollers (or “rolls”) positioned along the edges ofthe ribbon serve to draw, or pull the ribbon downward and/or apply anoutward tensioning force to the ribbon that helps maintain the width ofthe ribbon against an inward contraction. Some rolls may be rotated bymotors, whereas other rolls are freewheeling.

As the ribbon descends from the forming body, the molten material coolsand transitions from a viscous liquid state at the bottom of the formingbody, to a visco-elastic state and finally to a solid elastic state. Asused herein, the elastic state is generally considered to be when theglass material has reached a temperature below the glass transitiontemperature range. In some embodiments, the elastic state may beconsidered to be equivalent to a viscosity greater than 10¹³ poise. Whenthe ribbon has cooled to an elastic state, the ribbon may be scoredacross its width, and separated along the score line to produce aseparate glass sheet. In some embodiments separate glass sheets may becut in a single pass, without the need for scoring.

During the time the ribbon is in a liquid viscous state, stressesimposed on the molten material are immediately relieved. However, as theribbon cools and the viscosity increases, induced stresses are not soquickly relieved, until a temperature range is reached when inducedstresses and shape will be retained by the glass. This occurs over theglass transition temperature range. Both are sources of undesirableresidual stresses that can lead to warping of the final product. Forexample, residual stresses can be relieved by the ribbon (or separatedsheet) by causing the ribbon or sheet to change shape. The stress canthereby become wholly or partially relieved, but at the expense ofbuckling in the sheet. On the other hand, attempts to flatten the ribbonor sheet, such as by forcing the ribbon or sheet flat, result in aninduced stress in the glass. It is desirable, therefore, that during theperiod when stress and/or shape can be frozen into the glass that forcesimposed onto the glass ribbon be as consistent (and small) as possible.One source of force variation comes from the edge rolls. Note that forcevariation from the edge rolls could also result in variability to theglass thickness, and other product attributes. Experience has shown thatforce consistency is most important in achieving ultra low stress andhigh flatness requirements, for example, of LCD substrate sheets.

Although edge rolls may take different forms, a typical edge rollsubassembly comprises a pair of edge rolls that pinch or grip the ribbonedges. Consequently, pairs of edge rolls are positioned at oppositeedges of the ribbon so that for a particular vertical location along thelength of the ribbon (i.e. distance from the root), two pairs of edgerolls are used. Edge rolls may be driven, such as by electric orhydraulic motors, or edge rolls may be freewheeling. Edge rolls may bearranged to be substantially parallel and horizontal, or edge rolls maybe tilted so that the rotational axis of a roll is non-horizontal. Forexample, some edge rolls may be tilted in a vertical plane, whereasother edge rolls may be tilted in a horizontal plane. Edge rolls atopposing edges of the glass ribbon but adjacent the same surface mayshare a common shaft so that the shaft extends at least across the widthof the ribbon, or each edge roll may have its own, separate shaft thatextends only so far as necessary to position the edge roll contactsurface in an appropriate location to grip an enlarged portion of theribbon—the bead—at the edge of the glass ribbon. The edge roll shouldcontact the glass ribbon sufficiently to apply the desired force, butwith the understanding that the further inward toward the center of theribbon the roll contacts the sheet, the less pristine glass surface isavailable to produce pristine glass sheet. The edge roll is designed towithstand prolonged high temperatures, sometimes in excess of 800° C.,arising from contact with the glass ribbon, and preferably utilizes aceramic material. For example, edge rolls can be formed by stacking aplurality of disk-shaped ceramic fiber forms to produce a cylindricalroll body. The cylindrical roll body can then be secured at the end ofan edge roll shaft.

Each pair of edge rolls is designed to accommodate a varying gap betweenthe contact surfaces of the opposing rolls. For example, each edge rollmay not be perfectly concentric with the shaft to which it is attached,creating run out as the rolls rotate. As used herein, run out refers tothe radial position of the surface of the roll that contact the glassribbon as the roll rotates about an axis of rotation, generallyconsidered the rotational axis of the shaft to which the roll iscoupled. If the roll surface (the surface of the cylindrical body) isnot exactly concentric about the shaft axis of rotation, the distancebetween a point on the roll contact surface at a given location, forexample, at an arbitrary angular location, will vary as the contactsurface rotates with the shaft. This may occur, for example, if thecylindrical body is in fact not perfectly cylindrical, or if the shaftis not centered at the center of the cylindrical roll body. Further,machining tolerances to shaft straightness, and warp at operatingtemperature, contribute to such operational run-out. In addition, theedge rolls are designed to accommodate small fluctuations in thethickness of the ribbon edges by either pivoting or sliding against acounter force (counterweight). This movement of the edge rolls may occuras the ribbon descends between the edge roll pairs. In other words, theedge roll pairs must be able to separate, and then draw closer togetheragain as the rolls operate, either to accommodate the aforementionedshortcomings of the rolls themselves, and/or thickness variations of theribbon.

Preferably, the edge rolls are biased inward, toward the plane of theglass ribbon, by a biasing force. The biasing force is small enough toaccommodate outward movement (widening of the gap between the edge rollpair) from a given starting position that may be caused by the glassribbon for example, but strong enough to move the roll assembly backtoward the starting position when the force causing the outward movementis removed. For example, the edge rolls may include attached members(e.g. levers) arranged to pivot about a pivot point and allow the edgeroll shaft position (and the roll surface in contact with the ribbon) toaccommodate the fluctuation in roll shape or ribbon thickness.Counterweights may be used to apply sufficient force to the lever so theedge roll contact surfaces can grip the glass ribbon, yet still allowthe rolls to move outwardly in relation to the ribbon plane in responseto a varying roll eccentricity for example. However, other methods ofapplying a biasing force can be used, such as springs arranged to eitherpull or push the roll assembly along a predetermined path. An inherentissue with production roll systems used in the commercial production ofglass is that friction within the roll mechanism slides andbearings—along with friction within the seal plates that seal theopening through which the edge roll shaft traverses the enclosureenclosing the ribbon—resists the lateral (off-axis) motion of the shaftand imparts an undesired variable force that alters the actually pinchforce applied to the glass by the opposing rolls. Precise measurementsof forces experienced by the rolls in a direction perpendicular to thedirection of travel of the ribbon, for example, have shown a forcevariability of over 4.5 kg over a single roll rotation. Vertical pullingforces exerted on the ribbon can be similarly affected, and similarmeasurements have shown large variation in the pulling force applied tothe ribbon.

Summarizing, as the ribbon descends from the forming body, smallfluctuations in ribbon edge (bead) thickness, or for exampleeccentricity of the edge roll, causes movement of the edge roll in adirection transverse to the plane of the glass ribbon (perpendicular tothe direction of travel of the ribbon). A biasing force can be used tomaintain contact between the roll contact surface and the glass ribbonedge, but this biasing force has traditionally been “dumb” in the sensethat the biasing force has been supplied by a counterweight of a givenbut static magnitude, or by a spring or springs with a given springconstant. Ideally, the edge rolls should be free to react to variationsin the system variations (e.g. ribbon thickness changes, non-concentricrolls shaft surfaces, or other imperfections that cause run out in therotation of the rolls) without opposition. However, friction in thesystem opposes these accommodating motions. In the extreme, such as ifthe pulling roll pair was frozen in place and capable only of rotationof the roll contact surface (that is the opposing friction was infinitein magnitude), changes in the system would be acutely felt by theribbon. For example, if one or both edge rolls of an edge roll pair werenot concentric with their respective shafts, each revolution of a rollwould apply a cyclic force against the ribbon, in effect driving theroll into the ribbon. This cyclic force has a direct impact on thestress in the ribbon that varies with time.

To overcome the deficiencies noted above, an edge roll assembly isdescribed below that actively monitors tension at the edge roll shaft,and uses the measured tension as a feedback signal to modify the pinchforce exerted on the ribbon by an edge roll pair, thereby maintaining aconstant pinch force. In other embodiments, the pinch force may beactively varied in response to feedback from other process variables.Alternatively, the actual pinch force may be monitored and used as afeedback signal. While monitoring the pinch force is more direct, it isalso more difficult to implement as a practical matter. Nevertheless,pinch force monitoring may be used, for example, in the case when anedge roll shaft extends across the ribbon and couples to an opposingedge roll. These and other aspects are described in more detail below.

Shown in FIG. 1A is an exemplary fusion downdraw apparatus 10 comprisingforming body 12 including channel or trough 14 and converging formingsurfaces 16. Converging forming surfaces 16 meet at root 18, which formsa substantially horizontal draw line from which the molten glass isdrawn. By substantially horizontal what is meant is that the formingbody may in some cases be tilted longitudinally (end-to-end) or sidewaysto adjust the flow of molten glass over the top edges of the trough, butsuch tilting is generally slight, and may vary by only one or twodegrees or. Trough 14 is supplied with molten glass from a source, suchas a melting furnace and accompanying delivery piping (not shown), andthe molten glass overflows the walls of the trough and descends over theconverging forming surfaces of the forming body as separate streams. Theseparate streams of molten glass flowing over converging formingsurfaces 16 meet at root 18 and join each other to form glass ribbon 20.Glass ribbon 20 is drawn downward in direction 21 from root 18 and coolsas it descends from the root, transitioning from a viscous moltenmaterial to an elastic solid.

When glass ribbon 20 has reached a final thickness and viscosity in anelastic region of the ribbon, the ribbon is fully separated across itswidth to provide an independent glass sheet or pane. As molten glasscontinues to be supplied to the forming body, and the ribbon lengthens,additional glass sheets are similarly separated from the ribbon.

Lower enclosure 22 surrounds and encloses the upper reaches of ribbon 20below root 18 and connects with an upper enclosure 24 that housesforming body 12. Lower enclosure 22 protects the ribbon fromenvironmental influences (e.g. drafts, dust, etc.) and serves as aplatform on which various heating and/or cooling equipment may bepositioned to regulate the temperature of the ribbon as it travelsdownward. Other equipment, including the edge roll assemblies, may bemounted on, or interact with lower enclosure 22.

Edge roll assemblies 26 are positioned at predetermined verticallocations below root 18. Each edge roll assembly includes an edge roll28 and an edge roll shaft 30. Edge roll assemblies 26 may include drivenedge rolls used to apply a pulling force and/or tensioning force to theribbon and/or non-driven idler edge rolls that guide the ribbon and mayfurther help maintain a tension across the ribbon width. As describedabove, edge rolls positioned on one side of the ribbon may share acommon shaft across the width of the ribbon, or each edge roll may haveits own shaft. Edge rolls are typically arranged in pairs, where eachedge roll 28 of an edge roll pair is positioned on opposite sides of agiven edge of the ribbon from the other edge roll so the ribbon can bepinched between the opposing rolls of the roll pair as illustrated inFIG. 1B. In some embodiments, one edge roll of a pair of opposing edgerolls is stationary, while the other edge roll is free to move away andtoward the ribbon. That is, the stationary edge roll is free to rotate,but it does not change position away from or toward the ribbon.Additionally, edge roll pairs are themselves arranged in pairs, one pairof rolls per ribbon edge at a given vertical position.

Each edge roll assembly 26 includes support structures, bearings, andmeans of applying a driving force, if needed. Edge roll assemblies 26,including their operative structures, are subject to typicalmanufacturing tolerances. For example, edge rolls that contact ribbonedge portions 32 may not be exactly concentric with a respective shaft30. Or, an edge roll may be out of round (e.g. include a local flatnessor have an elliptical shape). Or an edge roll shaft may be not perfectlystraight. These factors may lead to a periodic lateral displacement ofan edge roll, and, like an out-of-round tire, may result in a periodicdisturbance in the ribbon each time the roll completes a revolution.This disturbance can manifest as a change in stress that can becomefrozen into the ribbon. In addition, the ribbon edges (or “beads”) areslightly bulbous, and their thickness may vary along the length of theribbon. In other words, the edge rolls of an edge roll pair should beconfigured to accommodate a varying gap between them.

Ideally, an edge roll assembly 26 is designed to accommodate theoperating movement of the edge rolls 28 and yet maintain a consistentpinch force on the glass ribbon between the rolls of an edge roll pair.However, in reality, friction within seal plates that seal an edge rollshaft penetration through the lower enclosure and/or other operatingmechanisms can cause the pinch force to vary. This variation in pinchforce, in turn, may cause the horizontal and vertical components of theroll forces imparted onto the glass ribbon to vary cyclically. Rollforce cycling can directly impact the resultant glass sheets, beingmanifest as stress or stress variability, warp or warp variability, oreven as variation in the glass thickness.

Shown in FIG. 2 is a view of a portion of the apparatus of FIG. 1B seenlooking toward one edge portion 32 of glass ribbon 20. A pair of opposededge roll assemblies 26 are depicted, each edge roll assembly comprisingan edge roll 28 coupled to edge roll support assembly 34 through an edgeroll shaft 30 and housing 36. Edge roll assemblies 26 may be positionedsuch that their respective edge rolls 28 can contact ribbon 20 at anyvertical location of the ribbon, including the viscous, visco-elastic orelastic portions, depending on the function of the edge roll. Edge rolls28 are typically cylindrically shaped bodies coupled to the shaft. Forexample, stacking a plurality of ceramic discs into a suitable rollshaft 30 may form the cylindrical body.

A passive bias force, e.g. bias force 38 shown in FIG. 2, is appliedagainst an edge roll support assembly 34 and, in combination with anopposing edge roll assembly, operates to pinch glass ribbon 20 betweenthe edge rolls 28 with a predetermined pinch force. The passive biasforce 38 may be, for example, a spring or deadweight coupled to the rollassembly. Both edge roll assemblies may be configured to move. However,as noted above, one edge roll assembly could also be stationary.Disturbances in the pinch force may result in movement of the edge rollslaterally outward in a direction generally transverse to the plane ofthe ribbon and against the passive bias force. To minimize frictionalresistance to movement of the edge rolls bearings used in the assembliescan be low friction bearings such as air bearings.

While the embodiment of FIG. 2 illustrates an apparatus that relies upontranslation of a pair of edge roll assemblies toward or away from theglass ribbon, it should be noted that movement of each edge roll supportneed not be simply translation. For example, each edge roll assembly maybe configured to swing about an axis so that an edge roll arcs away fromthe ribbon as shown in FIG. 3. In this instance, support assembly 34comprises support arms 40 and 42 and is designed to rotate about axis ofrotation 44, thereby causing support arm 40, and edge roll 28 coupledthereto, to move through arc 46. A spring or deadweight 48, can becoupled to either one of the support arms to apply a pressure againstglass ribbon 20 by one of the edge rolls 28. In the embodiment of FIG.3, the passive bias force is applied by gravity G through deadweight 48.

Preferably, the tension force across the ribbon and/or the pinch forceat one or more of the edge rollers may be measured in real time. Moreprecisely, reaction forces in the edge rolls shaft are measured and usedas a surrogate measure of the forces in the glass ribbon. In addition,the pinch force that each edge roller applies to the ribbon may also bemeasured, again in real time.

In broad outline, the tension and pinch forces in the shaft of each edgeroll and thus in the glass ribbon can be measured, for example, using aflexing member to which the edge roll is mounted. For example, asuitable measurement arrangement is disclosed in U.S. Patent ApplicationPublication 2010/0300214 filed on 27 May 2009 and described below. Theflexing member is designed to undergo small deflections in orthogonaldirections, i.e., the tension and pinch directions, when a load havingforce components in those directions is applied to the roll contactsurface by the ribbon. Displacement sensors detect the small deflectionsof the flexing member, at least one sensor being used for eachorthogonal axis along which forces are measured. Measuring thedeflections and then correlating them to deflections produced by knownloads can measure the orthogonal components of the force applied to theroll.

The flexing member is designed to substantially measure only loads alongspecified orthogonal axes even though loads may be applied in multipledirections. More particularly, the flexing member is designed so that ithas at least one portion that deflects in the direction of interest whena specific load is applied but has near zero deflection along the samedirection when transverse loads are applied. A displacement sensor isthen located to detect deflections of that portion of the flexingmember. In this way, the flexing member/displacement sensor combinationmeasures deflections of the flexing member from loads along thedirection of interest, but loads in transverse directions will have onlyminimal effect on the sensor.

The flexing member is also designed to be stiff enough to not adverselyaffect (upset) the glass forming process. In particular, it has beenfound that a flexing member having a high compliance can cause theforming process to become unstable. A stiff flexing member leads tosmall deflections, but in practice it has been found that accurate forcemonitoring can still be achieved provided a displacement sensor having ahigh resolution is used. Examples of suitable high-resolutiondisplacement sensors include inductive sensors, i.e., eddy currentsensors, piezoelectric sensors, strain gages, capacitive sensors, andoptical sensors. It should be noted that the stiffer the flexing member,the more sensitive the displacement sensor needs to be and vice versa. Aforce gage such as a load cell could also be used in place of adisplacement sensor. It should be noted that the load cell would notprovide a direct measurement of force since the load is being shared byeach of the webs, so a calibration of the load cell would be necessary.

In one embodiment, the apparatus includes a center beam (support member)surrounded by an outer frame. The center beam is connected to the outerframe by a series of thin webs, and the roller is attached to the centerbeam. The outer frame is fixed relative to the frame of the glass-makingmachine while the center beam is allowed to deflect relative to theouter frame due to the flexing of the thin webs.

When an axial load is applied to the roll contact surface via the glassmotion, the force is transferred through the webs into the fixed frame.The force causes the webs to deflect like a spring. The apparatusincludes a sensor that measures the relative deflection of the centerbeam with respect to the outer frame. By performing a calibrationprocess whereby a series of known loads are applied and the deflectionsrecorded and by then using interpolation, the load can be calculated forany measured deflection. In the case of thin flat webs, the load versusdeflection is linear, which allows for a simple calculation of the loadby using the slope of the force versus displacement calibration curve.When a normal load, as opposed to an axial load, is applied to the roll,the force is again transferred through the webs into the fixed frame. Inthis case, the motion of the center beam is a rotation, rather than atranslation. Again, a series of known loads are used to calibrate therotation and by using interpolation, the normal load can be calculatedfor any measured rotation. As with an axial load, for thin flat webs,the load versus deflection is linear.

To provide sufficient stiffness, the webs may be made of a materialhaving a high modulus of elasticity, such as a ceramic or a metal suchas stainless steel, e.g., 17-4 stainless steel. In addition to a highelastic modulus, the material preferably has a high yield strength towithstand the stresses induced in the webs. An estimate of the number ofwebs and the material properties appropriate for a specific applicationcan be obtained by, for example, modeling the webs as cantilevers. See,for example, Mechanical Analysis and Design by Arthur H. Burr, ElsevierNorth Holland, Inc., 1981, page 400. A finite element analysis can alsobe used for this purpose. In addition to a high elastic modulus and highyield strength, the material needs to be resistant to corrosion atelevated temperatures, such as those associated with glass makingequipment, since corrosion of the webs will change their stiffness andthus adversely affect the accuracy of the measurements made by themonitoring apparatus. Again, various ceramics and stainless steel canwithstand glass-making temperatures for extended periods of time withoutsubstantial deterioration. In one embodiment, the webs and fixed framecan be made from a single block of material, e.g., a single block ofstainless steel.

In certain embodiments, measurement of the axial load and the monitoringof the normal load are substantially independent of one another. Thatis, the cross talk between the two determinations, i.e., the error ineither determination as a result of the presence of the other force, isless than 1%. Thus, for example, if the device is calibrated using oneof the two forces and then the other force is applied, the change in themeasured values should be less than 1%.

FIGS. 4-13 illustrate an edge roll assembly 26 such as may be used inthe embodiment of FIG. 2. The edge roll assembly of FIGS. 4-13 comprisesa support assembly 34 and housing 36 suitable for measuring both thetension 50 and pinch force 52 resulting from contact of an edge roll 28with glass ribbon 20. In FIG. 4, glass ribbon 20 is assumed to be movingdownward in direction 21 so that shaft 30 turns counterclockwise as seenfrom the shaft (see reference number 54). It should be noted that in anopposing assembly (not shown) the shaft turns clockwise.

In overview, support assembly 34 includes a support member 56 (see, forexample, FIGS. 5, 6 and 10) that supports shaft 30 of the edge roll. Thesupport member undergoes linear displacement (see 78 in FIG. 10) inresponse to tension 50 and rotation (see reference number 58 in FIG. 13)in response to pinch force 52. As discussed above, in practice, thelinear displacement and rotation are detected and then converted toforce values by a calibration procedure in which known loads are appliedto the shaft and the resulting linear displacements and rotations aremeasured.

As shown most clearly in FIGS. 4, 5, and 9, support assembly 34 includestwo subassemblies 60 and 62 that in the embodiment shown are separablefrom one another. Subassembly 60 includes shaft 30 and edge roll 28,while subassembly 62 includes support member 56 and its associatedequipment for detecting linear displacements and rotations of thesupport member. By being separable, an edge roll can be replaced (e.g.,as part of regular maintenance), while leaving the roller's supportmember and its associated equipment in place. As shown most clearly inFIGS. 7 and 9, subassemblies 60 and 62 can be equipped with a femaledovetail portion 64 and a male dovetail portion 66, which allows the twosubassemblies to be separated and rejoined by a linear motion asillustrated by reference number 68 in FIG. 9. In addition tofacilitating assembly and disassembly, a dovetail joint when locked inplace by, for example, a moveable gibe (not shown), provides a solidconnection between the subassemblies as is desirable for making forcemeasurements. Other types of connections between the subassemblies can,of course, be used in place of a dovetail, e.g., the subassemblies canbe bolted together. Also, support assembly 34 can be constructed as aunitary device without subassemblies if desired.

In the embodiment shown, subassembly 60 includes housing 36 and plate 70which are connected to one another by pivot 72. The pivot allows shaft30 and edge roll 28 to be oriented at an angle with respect tohorizontal as illustrated in FIG. 4, while still leaving plate 70 in ahorizontal plane. The particular angle chosen for shaft 30 will dependon the application and the desired amount of tension to be applied tothe ribbon by the roller. Rather than using a pivot, shaft 30 and edgeroll 28 can be oriented at a fixed angle if desired. As discussed above,shaft 30 and edge roll 28 can be free turning or driven. In the lattercase, shaft 30 will be connected to a suitable drive apparatus 74 (FIG.1).

Subassembly 62 includes support member 56 and frame 76. During use,support member 56 is oriented parallel to the surface of ribbon 20 in aplane perpendicular to the ribbon (i.e., a horizontal plane for adowndraw process) so that the support member is responsive toacross-the-ribbon forces applied to shaft 30. In particular, asillustrated in FIG. 10, when a force is applied to shaft 30 whichincludes a force component in the across-the-ribbon direction, e.g., inthe direction of arrow 50 of FIG. 4, support member 56 undergoes alinear displacement in the direction of that force component, asillustrated by arrow 78 in FIG. 10. More particularly, in the embodimentshown, support member 56 undergoes such a linear displacement as aresult of elastic deformation of webs 80. For purposes of illustration,only eight webs are shown in FIG. 10 and the magnitude of the webs'deformation has been exaggerated. In practice, more than eight webscould typically be used, e.g., sixteen webs. Importantly, thedeformation of webs 80 is frictionless so that the presence offrictional forces does not interfere with the monitoring of the forceapplied to shaft 30. Although webs are preferred for support member 56,other supports can be used, e.g., springs of various configurations canbe employed for this purpose.

As illustrated in FIG. 11, the displacement of support member 56 isdetected using a sensor 82 and a sensor target 84, e.g., an inductivesensor (see above). One member of the sensor/target combination isattached to support member 56 and the other to frame 76. In FIG. 11, itis assumed sensor target 84 is attached to support member 56 andundergoes movement from an initial position 86 to a final position 88.By calibrating this displacement using known forces (see above), theforce applied to shaft 30 in the across-the-ribbon direction can bemonitored in real time by monitoring the relative movements between thesensor and its target.

In response to displacement of the sensor target, sensor 82 develops asignal that is received by controller 90 through line 92. Controller 90compares the signal received from sensor 82 and compares the receivedsignal to a predetermined set point. Controller 90 in turn generates acorrective signal that is received by actuator 94 over line 96. Actuator94 in turn is coupled to shaft 30 so that shaft 30, and thereby tensionor pinch force, can be changed in response to the corrective signal.Actuator 94 may be coupled to shaft 30, for example, through frame 76 ofsupport assembly 34, wherein a bearing assembly permits movement offrame 76 in a direction perpendicular to shaft 30. Actuator 94 maycomprise, for example, a compressed air or hydraulic cylinder, a linearservomotor or any other suitable actuating apparatus. Actuator 94 may beused alone, wherein actuator 94 supplies the total pinch force, oractuator 94 may be used on combination with a passive force applicator,such as one of the previously mentioned methods of applying a pinchforce (e.g. counterweights and/or springs). Controller 90 can be, forexample, a general-purpose computer, or any other type of processingunit capable of suitable signal processing.

In response to the corrective signal, actuator 94 is activated to moveframe 76 in a direction perpendicular to shaft 30, thereby increasing ordecreasing the pinch force as appropriate to maintain pinch force at thepredetermined value. For example, if the pinch force increases due to animperfection in the circularity of a contact surface, sensor 82 developsa force signal that alerts controller 90 to the change in pinch forcefrom a predetermined target pinch force. Controller 90 then develops acorrective signal proportional to the difference between thepredetermined target and the force signal that is received by actuator94. In response to the corrective signal, actuator 94 moves frame 76away from ribbon 20 in a direction perpendicular to the longitudinalaxis of shaft 30, thereby reducing the pinch force. This response can beused, for example, to maintain a constant pinch force.

In some embodiment, the edge rolls on one side of the ribbon can beconfigured with actuators that that respond to variations in pinch forceand/or shaft tension, whereas the opposing edge rolls on the other sideof the ribbon are fixed in position.

In other embodiments, controller 90 can be configured to apply a forcethat provides a total pinch force in accordance with a predeterminedschedule or in response to other draw conditions. For example, changesin ribbon temperature could be used by controller 90 to cause acompensating variation in force applied by actuator 94 and therebyvarying the total pinch force in response to ribbon temperature changes.

In addition to across-the-ribbon forces, the movement of support member56 can also be used to monitor forces applied to shaft 30 which includea component in a direction normal to the ribbon, e.g., in the directionof arrow 52 of FIG. 4. In this case, as illustrated in FIGS. 12 and 13,the motion of support member 56 is a rotation (see reference number 58),as opposed to a linear displacement. In these figures, arrow 96illustrates the force component normal to the ribbon. As best shown inFIG. 13, force 96 causes webs 80 to elastically deform during therotation. As with the linear displacement of FIG. 10, other meansbesides webs can be used to support member 56 so it can undergo rotationin response to a force having a component normal to the surface of theribbon.

However supported, the rotation of support member 56 is detected using asensor/target combination. As shown in FIG. 13, sensor 82 can be mountedon frame 76 and a target (not shown) can be mounted on an arm 98attached to support member 56. The arm serves to amplify the rotation ofthe support member, thus facilitating detection of the rotation. Bycalibrating the rotation of arm 98 using known forces (see above), theforce applied to shaft 30 in a direction normal to the ribbon can bemeasured in real time and the magnitude of the force communicated tocontroller 90. As described above, the controller compares the magnitudeof the measured force to a predetermined set point value, and develops acorrective signal if the measured value and the set point value are notequal. The corrective signal is received by actuator 94 that activatesand moves the edge roll to vary the pinch force.

It should be noted that as discussed above, the linear displacement ofsupport member 56 in response to a force component in theacross-the-ribbon direction is substantially independent of the rotationof the support member in response to a force component normal to theribbon, thus allowing these force components to be monitoredindependently of one another. The two components can be monitoredsimultaneously, sequentially, or periodically, as desired. Also, ratherthan detecting both linear displacements and rotations, only one of themotions of support member 56 can be detected if only one is of interestfor a particular application.

In some applications, it may be desirable to monitor the position ofsupport assembly 34 with respect to ribbon 20. In such a case, a target,e.g., an optical target, can be mounted on the outside surface of theassembly and its position detected as a function of time. As anotheralternative, a cable transducer can be attached to support assembly 34and used to monitor any changes in the location of the apparatus, e.g.,as a result of wear of edge roll 28 over time.

FIGS. 14-16 provide exemplary, non-limiting views of differentconfigurations in which the previously described embodiments can beconfigured. Seal plates are not shown for clarity. For example, FIG. 14illustrates a pair of edge rolls assemblies that are opposing across thethickness of the glass ribbon, and wherein actuator 94 (via linkagemember 100) is used to translate one edge roll assembly in a directionperpendicular to the longitudinal axis of shaft 30 (as indicate by arrow102). (See also FIG. 2)

FIG. 15 depicts a configuration where instead of translating one or bothshafts 30 in a direction perpendicular to the shaft(s), actuator 94 isused to vary an angle of the shaft in a horizontal plane. In theembodiment of FIG. 15, both roll shafts of the opposing edge rolls ismoved through an angle α in a horizontal plane.

In the embodiment of FIG. 16 (illustrating only a single edge roll of apair of opposing edge rolls), actuator 94 is used to move edge rollshaft 30 through an angle β in a vertical plane, thereby changing theangle of the shaft relative to an edge of ribbon 20.

It should be emphasized that the above-described embodiments of thepresent invention, particularly any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiments of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

What is claimed is:
 1. A method of making a glass ribbon comprising:producing a glass ribbon in a down draw glass making process, the glassribbon comprising a visco-elastic region; contacting the visco-elasticregion of the glass ribbon with opposing rollers, the opposing rollersapplying a pinch force in a thickness direction of the glass ribbon anda tension force in an across-the-ribbon direction of the glass ribbon;measuring a magnitude of the pinch force in the thickness direction ofthe glass ribbon or a magnitude of the tension force in theacross-the-ribbon direction of the glass ribbon and producing a forcesignal representative of the measured magnitude of the pinch force orthe tension force; comparing the force signal with a set point, andproducing a corrective signal representative of a difference between theforce signal and the set point; and using the corrective signal to drivean actuator that actively repositions at least one of the opposingrollers relative to the glass ribbon to vary the pinch force or thetension force applied to the glass ribbon by the opposing rollers sothat the applied pinch force or tension force is substantially equal tothe set point.
 2. The method according to claim 1, wherein at least aportion of the pinch force comprises a passive force.
 3. The methodaccording to claim 1, wherein the actuator translates a shaft of atleast one of the opposing rollers in a direction perpendicular to alongitudinal axis of the shaft.
 4. The method according to claim 1,wherein the actuator rotates a shaft of at least one of the opposingrollers through an angle lying in a horizontal plane.
 5. The methodaccording to claim 1, wherein the actuator rotates a shaft of at leastone of the opposing rollers through an angle lying in a vertical plane.6. The method according to claim 1, wherein a maximum peak to peakvariation in tension force in a lateral direction across a width of theribbon is less than 4.5 kg.
 7. A method of making a glass ribboncomprising: producing a glass ribbon in a down draw glass makingprocess; contacting an edge of the glass ribbon with opposing rollersthat applying a pinch force in a thickness direction of the glass ribbonor a tension force in an across-the-ribbon direction of on the glassribbon; sensing a magnitude of the pinch force in the thicknessdirection of the glass ribbon or a magnitude of the tension force in theacross-the-ribbon direction of the glass ribbon and producing a signalrepresentative of the sensed pinch force or the tension force; comparingthe produced signal with a set point, and producing a corrective signal;and using the corrective signal to drive an actuator that activelyrepositions at least one of the opposing rollers relative to the glassribbon to vary the pinch force or the tension force applied to the glassribbon to maintain a constant tension in the across-the-ribbon directionof the glass ribbon.
 8. The method according to claim 7, wherein theactuator moves a shaft of at least one roller of the opposing rollers ina direction perpendicular to a longitudinal axis of the shaft inresponse to the corrective signal.
 9. The method according to claim 7,wherein the actuator moves a shaft of the at least one roller through anangle lying in a horizontal plane in response to the corrective signal.10. The method according to claim 7, wherein the actuator moves a shaftof the at least one roller through an angle in a vertical plane inresponse to the corrective signal.
 11. The method according to claim 7,wherein a maximum peak to peak variation in the tension force in alateral direction across a width of the ribbon is less than 4.5 kg. 12.The method according to claim 7, wherein a maximum peak to peakvariation in the tension force in a lateral direction across a width ofthe ribbon is less than 3.0 kg.
 13. The method according to claim 7,wherein a maximum peak to peak variation in pinch force is less than 4.5kg.